Just possessing an unregistered firearm is not illegal but if you carry the unregistered firearm in public and get stopped by a police officer and the police officer then finds out that the firearm is unregistered then you will likely go to jail and then when your trial comes up you can be charged with a felony and could face up to 14 years in prison for carrying around the unregistered firearm.
If you plan to carry a firearm in public you need to get it registered in your name and have a license with you to carry the firearm as you can get some serious charges and some serious prison time depending on the state you're in.